Fix some bugs that blocked optimizations in the new IRs.

pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library tree_ir_builder;
 
 import '../dart2jslib.dart' as dart2js;
 import '../dart_types.dart';
 import '../elements/elements.dart';
 import '../cps_ir/cps_ir_nodes.dart' as cps_ir;
@@ skipping 180 matching lines @@
 
 
   List<Expression> translateArguments(List<cps_ir.Reference> args) {
     return new List<Expression>.generate(args.length,
          (int index) => getVariableReference(args[index]),
          growable: false);
   }
 
   List<Variable> translatePhiArguments(List<cps_ir.Reference> args) {
     return new List<Variable>.generate(args.length,
-         (int index) => getVariableReference(args[index]));
+         (int index) => getVariable(args[index].definition));

[asgerf, 2015/01/15 12:08:00]

getVariableReference() increases readCount.
getVariable() does not.

readCount is now set in addAssignment when we actually do the assignment.

This avoids spurious readCount in cases where the assignment is unnecessary
because register allocation merged the two variables.

[sigurdm, 2015/01/15 12:21:03]

Maybe put this explanation in a comment in the code.

[asgerf, 2015/01/15 13:05:37]

Done.

   }
 
   Statement buildContinuationAssignment(
       cps_ir.Parameter parameter,
       Expression argument,
       Statement buildRest()) {
     Variable variable = getVariable(parameter);
     Statement assignment;
     if (variable == null) {
       assignment = new ExpressionStatement(argument, null);
@@ skipping 28 matching lines @@
       }
       List<int> list = rightHand[arg];
       if (list == null) {
         rightHand[arg] = list = <int>[];
       }
       list.add(i);
     }
 
     Statement first, current;
     void addAssignment(Variable dst, Variable src) {
+      ++src.readCount;
+      // `dst.writeCount` will be updated by the Assign constructor.
       if (first == null) {
         first = current = new Assign(dst, src, null);
       } else {
         current = current.next = new Assign(dst, src, null);
       }
     }
 
     List<Variable> assignmentSrc = new List<Variable>(parameters.length);
     List<bool> done = new List<bool>(parameters.length);
     void visitAssignment(int i) {
@@ skipping 284 matching lines @@
     // visited.
     internalError(CURRENT_ELEMENT_SPANNABLE, 'Unexpected IR node: $node.');
     return null;
   }
 
   Expression visitIsTrue(cps_ir.IsTrue node) {
     return getVariableReference(node.value);
   }
 }